The present invention relates generally to a device for capacitively sensing media thickness. More specifically, the present invention relates to a device for sensing the thickness of a media by using a variable capacitance capacitor that includes electrodes having a variable gap disposed therebetween and a dielectric of gas disposed in the variable gap. The thickness of the media is determined by measuring the capacitance between the electrodes. The capacitance between the electrodes is determined by the distance between the electrodes and the dielectric constant of the gas disposed in the variable gap and not by the dielectric properties of the media whose thickness is being measured.
Media thickness sensors are employed in media handling systems such as inkjet printers, laser printers, photocopying machines, document scanners, facsimile machines, and film production processes, just to name a few. Media thickness sensors can be implemented using a variety of technologies, such as mechanical sensors, optical sensors, and capacitive sensors.
A typical mechanical thickness sensor includes at least one member that is connected to a measurement circuit. The member is operative to engage a surface of a media whose thickness is to be measured. Contact between the member and the media results in the member being displaced. The measurement circuit measures the displacement of the member and generates a signal indicative of the thickness of the media. One disadvantage of mechanical thickness sensors is that the complexity and cost of the mechanical elements can be prohibitive in applications that require low cost, mechanical simplicity, and compact size. Another disadvantage of mechanical thickness sensors is that the mechanical elements are prone to failure and can require periodic maintenance and adjustment in order to maintain peak operational efficiency and measurement accuracy.
An optical thickness sensor can include a light source, such as a light-emitting diode (LED) and a light-sensitive element, such as a photodiode. The LED can be electrically driven by a power source, such as a constant-current source, and an output of the photodiode can be connected to a measurement circuit. Typically, the LED and the photo diode are positioned so that a beam of light from the LED is incident on the photodiode. The beam is either reflected off of a surface of the media whose thickness is to be measured or the beam is transmitted through the media. The output signal from the photodiode is proportional to the intensity of light incident on the photodiode and is indicative of the thickness of the media. One major disadvantage of the optical thickness sensor is that the accuracy of the thickness measurement is highly dependent on the optical properties of the media being measured. For example, if the media is opaque, then little or no light from the LED will reach the photodiode. Subsequently, the optical thickness sensor will not accurately measure the thickness of opaque media. Similarly, if the media is highly reflective of light or is translucent (clear), then there will be little or no variation in the intensity of light incident on the photodiode. Resulting will be inaccurate thickness measurements for reflective or clear media of different thicknesses. Another type of optical sensor measures thickness by reflecting light off of the media or a reflective surface in contact with the media into a light sensor. The position of the reflected light on the light sensor is indicative of the thickness of the media. Disadvantages of reflective sensors include electrical and mechanical complexity, high cost, and precision alignment of the optical components. A second disadvantage of the optical thickness sensor is that the sensor is often tuned to measure the thickness of a narrow range of media types or only of a specific type of media. Resulting, is a lack of flexibility in measuring a wide variety of media types. For example, the optical thickness sensor may be tuned to measure the thickness of white printer paper only. A third disadvantage of the optical thickness sensor is that if the media is of substantially uniform thickness, but has variable optical properties, then those variations in optical properties can result in inaccurate thickness measurements.
The use of capacitive elements to measure the thickness of a media is well known in the art. In a typical capacitive thickness sensor, opposed electrodes are urged into contact with opposed surfaces of a media whose thickness is to be measured. The media is disposed intermediate between the electrodes and the capacitance measured between the electrodes is a function of the dielectric properties of the media, the area of the electrodes, and the distance between the electrodes, so that the capacitance is determined by the following equation:
C=(∈m*A)÷d
Where:
C=The capacitance measured between the electrodes;
∈m=The dielectric constant of the media;
A=The area of the electrodes; and
d=The spacing between the electrodes.
The change in capacitance between the electrodes can be sensed by appropriate electronic circuitry that produces a signal that is indicative of the thickness of the media. The sensing circuitry is well known in the art. For example, the electrodes can be in electrical communication with a constant current source. The voltage potential measured between the electrodes will increase linearly with time until a reference voltage is reached. The amount of time required for the voltage to reach the reference voltage is proportional to the thickness of the media. For instance, for thicker media, the time it takes to reach the reference voltage is shorter. Accordingly, based on the equation above, both the capacitance and the time it takes the voltage to reach the reference voltage decrease with increasing distance (thickness) of the media d.
In FIG. 1, a prior art capacitive thickness sensor 200 is shown. The sensor 200 includes electrically conductive plates 203 and 205 that are disposed opposite each other and are in physical contact with opposite surfaces 202 and 204 respectively, of a media 201 whose thickness d is to be measured. The plate 205 can be disposed on a support structure 209 that is operative to contain the plate 205 and can also serve as a surface upon which the media rests during the thickness measurement. In a typical application, the plates 203 and 205 will have identical surface areas a. The plate 203 is urged into contact with the media 201 by a biasing means 211 that is attached to a stationary support element 207. The biasing means 211 can be a spring, for example. Depending on the thickness of the media 201, the plate 203 is displaced 213 in response to the thickness of the media 201 so that when the media 201 is urged between the plates 203 and 205 the distance between the plates 203 and 205 is substantially equal to the thickness d of the media 201. Electrical connections 215 and 217 electrically communicate the plates 203 and 205 respectively to a capacitance sensing unit 221. The capacitance sensing unit 221 can use any method, including the one discussed above, to measure the capacitance between the plates 203 and 205. An output signal 222 from the capacitance sensing unit 221 can be communicated to a control unit 223 that uses the signal to compute the thickness of the media 201. For instance, if the media 201 has a dielectric constant ∈m, a know area a for the plates 203 and 205, and the output signal 222 is indicative of a capacitance value of C, then the above equation can be used by the control unit 223 to compute the distance d between the plates 203 and 205, wherein the distance d is substantially equal to the thickness of the media 201. In the prior art capacitive thickness sensor 200, the capacitance measured between the plates 203 and 205 is inversely proportional to the distance d between the plates 203 and 205.
One disadvantage of the prior art capacitive thickness sensor 200 is that the accuracy of the thickness measurement will vary with changes in the dielectric constant ∈m of the media 201 due to environmental conditions such as temperature and humidity and due to local variations in the dielectric constant ∈m of the media 201 at different portions of the media 201. Additionally, different types of media or different brands of media have a significant impact on changes to the dielectric constant ∈m of the media. Subsequently, the thickness measurement is not an absolute one, rather it is a derived measurement that is directly dependent on the dielectric constant ∈m. Another disadvantage of the prior art capacitive thickness sensor 200 is that the plates 203 and 205 must be maintained in fixed relation to each other so that the distance d between the plates 203 and 205 does not vary, i.e. the plates 203 and 205 must be maintained in parallel relation to each other. The biasing means 211 is operative to urge the plates 203 and 205 into contact with the media 201 with sufficient force to establish the parallel relation between the plates 203 and 205 and to ensure the entire surface area a of the plates 203 and 205 is in snug contact with the media 201. However, that force can result in the plates 203 and 205 compressing (squishing) the media so that the actual thickness of the media is reduced by the compressive force of the plates 203 and 205. Resulting is an inaccurate thickness measurement. Additionally, if the media 201 is in motion while the plates 203 and 205 are in contact with the media 201, that motion can result in heat generated by friction between the plates 203 and 205 and the media 201. The dielectric constant ∈m can vary with temperature thereby affecting the accuracy of the thickness measurements. Moreover, motion between the plates 203 and 205 can result in wear and damage to the media 201 and the plates 203 and 205. For example, if the media 201 is a film, excessive pressure from the plates 203 and 205 could scratch the film.
Changes in the dielectric constant ∈m due to environmental conditions such as temperature and humidity are also addressed by the prior art thickness sensors as illustrated in FIG. 2. In FIG. 2, a prior art reference capacitor 210 includes plates 233 and 235 that are contained in a housing 231 that is operative to disposed the plates 233 and 235 opposite each other and in fixed relation to each other, thereby defining a constant reference distance dR between the plates 233 and 235. A dielectric material 237 is disposed between the plates 233 and 235 and is in contact with the plates 233 and 235. The dielectric material 237 has a predetermined dielectric constant ∈R. The plates 233 and 235 are connected to the capacitance sensing unit 221 by electrically conductive leads 225 and 227 respectively. Additionally, the electrical connections 215 and 217 as discussed above in reference to FIG. 1, are also connected to the capacitance sensing unit 221. The capacitance sensing unit 221 measures the capacitance of the reference capacitor 210 and uses the measured capacitance to either compensate for or to nullify the variations in the capacitance measured between the plates 203 and 205 (see FIG. 1) due to the above mentioned environmental conditions. One disadvantage to using the reference capacitor 210 is that it adds to the complexity of the capacitive thickness sensor 200 of FIG. 1. For instance, the use of the reference capacitor 210 can require additional circuitry for the capacitance sensing unit 221. In some manufacturing operations, the housing 231 can be adapted to allow for manual adjustments to the reference distance dR by using a micrometer, for example, to adjust the spacing between the plates 233 and 235 so that the value of the reference capacitor 210 is known value before thickness measurements are made using the capacitive thickness sensor 200. A second disadvantage to using the reference capacitor 210 is that the relationship between the dielectric constant of the calibrant (∈R) and variations in the dielectric constant of the media ∈m may not be a clearly understood relationship. Consequently, the difference (xcex94) between ∈R and ∈m may not correspond between different media.
Therefore, there is a need for a capacitive thickness sensor that is mechanically simple, is low cost, requires no maintenance, can sense media thickness irrespective of the dielectric or optical properties of the media, can accurately sense the thickness of a wide variety of media types, can compensate for environmental conditions without the use of additional sensors, eliminates damage to the media and to the sensor, and does not squish the media.
The present invention is an improvement in the design of capacitive thickness sensors that can be used for measuring the thickness of a media. The aforementioned limitations and disadvantages of various types of media thickness sensors are addressed by the present invention.
The present invention is mechanically simple, can be constructed from a few parts, and can be made from low cost materials such as plastic, for example. Moreover, the present invention does not require maintenance, calibration, or adjustments.
The dielectric and/or optical properties of the media to be measured and any inconsistencies in those properties have no effect on the accuracy of the media thickness measurements of the present invention because the electrodes of the present invention do not contact the media to be measured and are positioned away from the media to be measured so that the dielectric properties of the media do not interact with the electrodes.
Additionally, because the electrodes of the present invention do not contact the media, the amount of force required to effectuate measuring the thickness of the media is reduced, thereby minimizing compression (squishing) of the media that can result in inaccurate thickness measurements. Another benefit of the present invention is that damage to the media and to the electrodes is eliminated because the electrodes do not contact the media.
The present invention can be used to measure media thickness and to compensate for environmental conditions using the same set of electrodes. Therefore, both the mechanical complexity of using additional components to implement environmental compensation and the additional circuitry required are eliminated by the present invention.
Broadly, the present invention is embodied in a device for capacitively sensing media thickness that includes first and second supports that are moveable relative to each other and a variable capacitance capacitor comprising first and second electrodes that have a variable gap disposed between the electrodes and a dielectric medium disposed in the variable gap. The dielectric medium can be a vacuum or a gas. The first and second electrodes are disposed on a portion of the first and second supports respectively and are disposed opposite each other in substantially facing relation. The electrodes are spaced apart by a first distance.
When a media whose thickness is to be measured and the supports are urged into contact with one another, the electrodes are displaced to a second distance. The media is not disposed in the variable gap and is not in contact with the electrodes so that the capacitance of the variable capacitor when the electrodes are in the second position is determined by the distance between the electrodes in the second position and the dielectric constant of the dielectric medium.
In another embodiment of the present invention, the device includes a reference stop positioned to maintain a consistent spacing between the electrodes when the electrodes are spaced apart by the first distance so that the capacitance of the capacitor when the electrodes are spaced apart by the first distance is determined primarily by the dielectric constant of the dielectric medium in the variable gap and not by variations in spacing between the electrodes. The consistent spacing between the electrodes can be used to electronically measure a reference capacitance between the electrodes that can be used to compensate for environmental conditions including temperature and humidity that can affect the accuracy of the capacitance electronically measured between the electrodes when the electrodes are spaced apart by the second distance.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.